docs: Fix grammatical error Update README.md

[annwag]

Description:

While reviewing the Introduction section of the guide, I noticed a minor grammatical error in the sentence:

Current:

The target audience of this guide are developers of nearcore itself.

The issue lies in the use of "are" instead of "is." The subject "audience" is singular, so the correct verb form should be "is."

Fixed:

The target audience of this guide is developers of nearcore itself.

This update ensures grammatical accuracy and improves the readability of the text.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.51%. Comparing base (4b2dd8e) to head (96dadce).
Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12669      +/-   ##
==========================================
+ Coverage   70.48%   70.51%   +0.02%     
==========================================
  Files         845      845              
  Lines      172258   172258              
  Branches   172258   172258              
==========================================
+ Hits       121421   121465      +44     
+ Misses      45747    45698      -49     
- Partials     5090     5095       +5     

Flag	Coverage Δ
backward-compatibility	0.16% <ø> (ø)
db-migration	0.16% <ø> (ø)
genesis-check	1.36% <ø> (?)
linux	69.35% <ø> (+0.02%)	⬆️
linux-nightly	70.11% <ø> (+<0.01%)	⬆️
pytests	1.66% <ø> (+0.19%)	⬆️
sanity-checks	1.47% <ø> (ø)
unittests	70.34% <ø> (+<0.01%)	⬆️
upgradability	0.20% <ø> (?)

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