doc/../43_schema.txt: remove unnecessary word

For consistency with https://github.com/cuelang/cue/blob/master/doc/tutorial/basics/2_types/55_defs.txt, this removes an unnecessary "a". Without the extra word, it also arguably _reads_ better, since readers aren't necessarily "pronouncing" hashtag/pound-sign in their minds as they read the text.

Closes #677
#677

GitOrigin-RevId: e57be73
Change-Id: I1bd23ee6f4fa17f9fe050965cb609d1d3aa6a2e7
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8364
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

doc/tutorial/basics/0_intro/43_schema.txt

@@ -8,7 +8,7 @@ description = ""
 -- text.md --
 In CUE, schemas are typically written as Definitions.
 A definition is a field which identifier starts with
-a `#` or `_#`.
+`#` or `_#`.
 This tells CUE that they are to be used for validation and should
 not be output as data; it is okay for them to remain unspecified.