add += to assignment operators

Author: dustypomerleau

editors/code/rust.tmGrammar.json

@@ -693,7 +693,7 @@
                 {
                     "comment": "assignment operators",
                     "name": "keyword.operator.assignment.rust",
-                    "match": "(-=|\\*=|/=|%=|\\^=|&=|\\|=|<<=|>>=)"
+                    "match": "(\\+=|-=|\\*=|/=|%=|\\^=|&=|\\|=|<<=|>>=)"
                 },
                 {
                     "comment": "single equal",