fixed some typos (#1814)

doc/how_to/contribute.md

@@ -138,7 +138,7 @@ make the-markdown-to-make.md
 - Add the generated figure to the ```dmlc/web-data``` repo.
   - If you already cloned the repo to doc, this means a ```git add```
 - Create PR for both the markdown  and ```dmlc/web-data```
-- You can also build the document locally by typing the followig command at ```doc```
+- You can also build the document locally by typing the following command at ```doc```
 ```bash
 make html
 ```

doc/how_to/index.md

@@ -6,7 +6,7 @@ This page contains guidelines to use and develop mxnets.
 - [How to Install XGBoost](../build.md)
 
 ## Use XGBoost in Specific Ways
-- [Parameter tunning guide](param_tuning.md)
+- [Parameter tuning guide](param_tuning.md)
 - [Use out of core computation for large dataset](external_memory.md)
 
 ## Develop and Hack XGBoost