Remove some parens in README grammar
Remove some parens in README grammar

Recent versions of node can parse arrow functions that return un-parenthesized regex literals.
tree-sitter/tree-sitter-cli,tree-sitter/tree-sitter-cli,tree-sitter/node-tree-sitter-compiler,tree-sitter/tree-sitter-cli,maxbrunsfeld/node-tree-sitter-compiler,tree-sitter/node-tree-sitter-compiler,maxbrunsfeld/node-tree-sitter-compiler,maxbrunsfeld/node-tree-sitter-compiler,tree-sitter/node-tree-sitter-compiler,tree-sitter/node-tree-sitter-compiler,tree-sitter/tree-sitter-cli