doc/tutorials/basics: typo
doc/tutorials/basics: typo

Closes #263

Change-Id: I70fe6b0982a8890100195cbd0f31138560667a15
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/4663
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>

cuelang/cue