core: fix build with Null backend

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

build.zig

@@ -261,7 +261,7 @@ pub fn build(b: *std.Build) !void {
             .install_subdir = "docs",
         });
         const docs_step = b.step("docs", "Generate docs");
-        docs_step.dependOn(&install_docs.step);          
+        docs_step.dependOn(&install_docs.step);
     }
 }