fix(doc): numbering error in README (apache#409)

This PR fixes a minor numbering issue in the README file discovered
while learning the iceberg-cpp project.

Author: slfan1989

mkdocs/README.md

@@ -22,6 +22,6 @@
 To build the documentation:
 
 1. Install [Doxygen](https://www.doxygen.nl/).
-1. From this directory, run `doxygen`.
+2. From this directory, run `doxygen`.
 
 The output will be in `./build/html`.