syntax: go: fix word boundary before dot in number literal

The fragment "\<\." can never match anything because \< matches the
beginning of a word but "." is not a word character.

Replace \< with \B (the empty string not at the edge of a word).
Signed-off-by: Tom Levy <tomlevy93@gmail.com>

syntax/go.nanorc

@@ -26,7 +26,7 @@ color brightcyan "//[[:space:]]*\+build[[:space:]]+(([a-zA-Z_0-9]+[[:space:]]*)+
 # Literals.
 color red "\<[0-9]+\.[0-9]*([Ee][+-][0-9]+)?i?\>"
 color red "\<[0-9]+[Ee][+-][0-9]+i?\>"
-color red "\<\.[0-9]+([Ee][+-][0-9]+)?i?\>"
+color red "\B\.[0-9]+([Ee][+-][0-9]+)?i?\>"
 color red "\<[0-9]+i\>"
 color red "\<[1-9][0-9]*\>"
 color red "\<0[0-7]*\>"