Fix citation format in JOSS paper

Remove square brackets from explicit citations per JOSS guidelines. Only the Epstein citations needed to be changed since they were explicit references, while all other citations were used parenthetically and already had correct formatting.

paper/paper.md

@@ -55,7 +55,7 @@ preferred-citation: article
 Mesa is an open-source Python framework for agent-based modeling (ABM) that enables researchers to create, analyze, and visualize agent-based simulations. Mesa provides a comprehensive set of tools and abstractions for modeling complex systems, with capabilities spanning from basic agent management to sophisticated representation of spaces within which agents interact. First introduced in 2014 [@masad2015], with updates published in [@kazil2020], this paper presents Mesa in its current version (3.1.1) as of late 2024.
 
 # Statement of need
-Agent-based models (ABMs), or artificial societies, are composed of autonomous heterogeneous agents interacting locally with other agents. These interactions give rise to emergent phenomena. The aggregate dynamics of a system under study emerge from these local interactions [@epstein_axtell_1996] [@epstein1999]. This type of modeling quickly grew more sophisticated, requiring frameworks to execute them. This led to the establishment of [NetLogo](https://ccl.northwestern.edu/netlogo/) and [MASON](https://cs.gmu.edu/~eclab/projects/mason/).
+Agent-based models (ABMs), or artificial societies, are composed of autonomous heterogeneous agents interacting locally with other agents. These interactions give rise to emergent phenomena. The aggregate dynamics of a system under study emerge from these local interactions @epstein_axtell_1996 @epstein1999. This type of modeling quickly grew more sophisticated, requiring frameworks to execute them. This led to the establishment of [NetLogo](https://ccl.northwestern.edu/netlogo/) and [MASON](https://cs.gmu.edu/~eclab/projects/mason/).
 
 However, before Mesa, there was no modern Python-based framework for ABMs that integrated with the scientific Python ecosystem. Since it’s creation in 2014, Mesa has been applied to modeling everything from economics and sociology to ecology and epidemiology and has been cited in more than 500 papers and 800 authors. Today, Mesa advanced usability and stabilized functionality with its most recent major release (3+). These features include enhanced management of agents, data collection advancements, improved visualization framework, and making it easier for researchers to create and analyze complex simulations.