docs: fix gcode asciidoc markup

Before this commit, the g-code.html document lacks a document <title>.
Looks like asciidoc needs the document title on the first line of the
txt input.

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>

docs/src/gcode/g-code.txt

@@ -1,6 +1,5 @@
-[[cha:g-codes]] (((G Codes)))
-
 = G Codes
+[[cha:g-codes]] (((G Codes)))
 
 :ini: {basebackend@docbook:'':ini}
 :hal: {basebackend@docbook:'':hal}