template: Fix off-by-one error in the class method arguments

The arguments array does not contain the instance argument, which means
it starts from zero.
3 jobs for class-methods in 1 minute and 45 seconds (queued for 6 seconds)
Status Name Job ID Coverage
  Check
passed flake8 #1437607

00:01:34

failed mypy #1437608
allowed to fail

00:01:25

passed reuse #1437609

00:01:45

 
Name Stage Failure
failed
mypy Check
gidocgen/gir/parser.py:982: error: Argument "get_type" to "GType" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:1004: error: Argument "name" to "Union" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:1004: error: Argument "symbol_prefix" to "Union" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:1004: error: Argument "ctype" to "Union" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:1006: error: Argument 1 to "set_version" of "GIRElement" has incompatible type "Optional[str]"; expected "str"
gidocgen/utils.py:809: error: Need type annotation for 'found_programs' (hint: "found_programs: Dict[<type>, <type>] = ...")
Found 205 errors in 3 files (checked 17 source files)
Cleaning up file based variables
ERROR: Job failed: exit code 1