fix incorrect position of topic

Right now "Clarification of build command's stdout" topic
shows up under the "Environment Variables" which is not correct.
This commit aims to fix this problem.

Signed-off-by: ozkanonur <[email protected]>

Author: onur-ozkan

src/doc/rustc-dev-guide/src/building/bootstrapping.md

@@ -449,7 +449,7 @@ This is an incomplete reference for the outputs generated by bootstrap:
 
 `--stage=2` stops here.
 
-### Clarification of build command's stdout
+## Clarification of build command's stdout
 
 In this part, we will investigate the build command's stdout in an action
 (similar, but more detailed and complete documentation compare to topic above).
@@ -468,17 +468,17 @@ Building stage1 tool rust-analyzer-proc-macro-srv (x86_64-unknown-linux-gnu)
 Building rustdoc for stage1 (x86_64-unknown-linux-gnu)
 ```
 
-#### Building stage0 {std,compiler} artifacts
+### Building stage0 {std,compiler} artifacts
 
 These steps use the provided (downloaded, usually) compiler to compile the
 local Rust source into libraries we can use.
 
-#### Copying stage0 {std,rustc}
+### Copying stage0 {std,rustc}
 
 This copies the library and compiler artifacts from Cargo into
 `stage0-sysroot/lib/rustlib/{target-triple}/lib`
 
-#### Assembling stage1 compiler
+### Assembling stage1 compiler
 
 This copies the libraries we built in "building stage0 ... artifacts" into
 the stage1 compiler's lib directory. These are the host libraries that the