Fix markup in "Operators" section

Some of the operators ('+' and '++') were being treated as asciidoctor
markup, so they were disappearing and some of the characters that were
supposed to be markup were appearing in their place. Fix by using the
builtin macro for '+' instead of the character.

Fixes Public GLSL issue KhronosGroup#214.

core.txt

@@ -9580,8 +9580,8 @@ endif::ESSL[]
     Results are undefined if one or both operands are negative.
     The operator modulus (*%*) is not defined for any other data types
     (non-integer types).
-  * The arithmetic unary operators plus (*+*), negate (*-*), post- and pre-increment and
-    decrement (*--* and *++*) operate on integer or floating-point values
+  * The arithmetic unary operators plus (*{plus}*), negate (*-*), post- and pre-increment and
+    decrement (*--* and *{plus}{plus}*) operate on integer or floating-point values
     (including vectors and matrices).
     All unary operators work component-wise on their operands.
     These result with the same type they operated on.