Fix typo in Generating-sources.md
First `custom_target` example was missing a closing `'`.
@@ -19,7 +19,7 @@ mycomp = find_program('mycompiler')
Custom targets can take zero or more input files and use them to generate one or more output files. Using a custom target, you can run this compiler at build time to generate the sources:
```meson
-gen_src = custom_target('gen-output,
+gen_src = custom_target('gen-output',
input : ['somefile1.c', 'file2.c']
output : ['out.c', 'out.h']
command : [mycomp, '@INPUT@',
