Remove uses of deprecated callconv aliases

doc/langref/export_builtin.zig

@@ -2,6 +2,6 @@ comptime {
     @export(&internalName, .{ .name = "foo", .linkage = .strong });
 }
 
-fn internalName() callconv(.C) void {}
+fn internalName() callconv(.c) void {}
 
 // obj

doc/langref/test_defining_variadic_function.zig

@@ -2,7 +2,7 @@ const std = @import("std");
 const testing = std.testing;
 const builtin = @import("builtin");
 
-fn add(count: c_int, ...) callconv(.C) c_int {
+fn add(count: c_int, ...) callconv(.c) c_int {
     var ap = @cVaStart();
     defer @cVaEnd(&ap);
     var i: usize = 0;

doc/langref/test_functions.zig

@@ -35,7 +35,7 @@ fn abort() noreturn {
 
 // The naked calling convention makes a function not have any function prologue or epilogue.
 // This can be useful when integrating with assembly.
-fn _start() callconv(.Naked) noreturn {
+fn _start() callconv(.naked) noreturn {
     abort();
 }
 

doc/langref/test_opaque.zig

@@ -2,7 +2,7 @@ const Derp = opaque {};
 const Wat = opaque {};
 
 extern fn bar(d: *Derp) void;
-fn foo(w: *Wat) callconv(.C) void {
+fn foo(w: *Wat) callconv(.c) void {
     bar(w);
 }