Fix typo 'DWASM_ENALBE_WASI' in WAMR build instructions (#155)

doc/build_wamr.md

@@ -26,7 +26,7 @@ make
 Note:
 The WASI feature is enabled by default, if we want to disable it, please run:
 ``` Bash
-cmake .. -DWASM_ENALBE_WASI=0
+cmake .. -DWASM_ENABLE_WASI=0
 ```
 
 Linux SGX (Intel Software Guard Extention)