fix: single ' parsed as (optionlink) error

Single apostrophes are found in various :help documents such as ":help
'fillchars'". They should be treated as plaintext.

corpus/optionlink.txt

@@ -28,6 +28,40 @@ world 'hello' world
         (word))
       (word))))
 
+================================================================================
+NOT optionlink: ' followed by whitespace
+================================================================================
+'fillchars'
+	  stl		' ' or '^'	statusline
+	  wbr		' '		windowbar
+          tricky:	' 'yes'
+
+
+--------------------------------------------------------------------------------
+
+(help_file
+  (block
+    (line
+      (optionlink
+        (word)))
+    (line
+      (word)
+      (word)
+      (word)
+      (word)
+      (word)
+      (word))
+    (line
+      (word)
+      (word)
+      (word)
+      (word))
+    (line
+      (word)
+      (word)
+      (optionlink
+        (word)))))
+
 ================================================================================
 NOT optionlink #7 #14
 ================================================================================

grammar.js

@@ -62,7 +62,7 @@ module.exports = grammar({
     // Explicit special cases: these are plaintext, not errors.
     _word_common: () => choice(
       // NOT optionlink: single "'".
-      /[\t ]'[\t ]/,
+      /'[\t ]/,
       // NOT optionlink: contains any non-lowercase char.
       seq("'", token.immediate(/[^'\n\t ]*[^'a-z\n\t ][^'\n\t ]*/), token.immediate("'")),
       // NOT optionlink: single char surrounded by "'".