gitmodules: Replace taps with spaces

.gitmodules

@@ -41,5 +41,5 @@
     path = externals/Vulkan-Headers
     url = https://github.com/KhronosGroup/Vulkan-Headers.git
 [submodule "externals/zstd"]
-	path = externals/zstd
-	url = https://github.com/facebook/zstd
+    path = externals/zstd
+    url = https://github.com/facebook/zstd