Update Go layer to reflect vim-go var name change

vim-go renamed go_highlight_methods to go_highlight_function_calls
recently to address confusion with the way the existing variable names
behaved [1]. This change will re-enable the higlighting of function and
method _invocations_.

[1] fatih/vim-go#1557 (comment)

autoload/SpaceVim/layers/lang/go.vim

@@ -39,7 +39,7 @@ endfunction
 
 function! SpaceVim#layers#lang#go#config() abort
   let g:go_highlight_functions = 1
-  let g:go_highlight_methods = 1
+  let g:go_highlight_function_calls = 1
   let g:go_highlight_structs = 1
   let g:go_highlight_operators = 1
   let g:go_highlight_build_constraints = 1