.pytool: Fix python command error in self introduction doc

Fix misspelling of word "Prerequisites".

In the step 5 of the Prerequisites section, the python command was missing
the "-r" option, what the command really meant was to install from the given
requirements file, so fix that.

Cc: sean.brogan@microsoft.com
Cc: Bret.Barkelew@microsoft.com
Cc: michael.d.kinney@intel.com
Cc: gaoliming@byosoft.com.cn

Signed-off-by: Chao Li <lichao@loongson.cn>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>

.pytool/Readme.md

@@ -82,7 +82,7 @@ easily and consistently running locally and in a cloud ci environment.  To do
 that a few steps should be followed.  Details of EDKII Tools can be found in the
 [docs folder here](https://github.com/tianocore/edk2-pytool-extensions/tree/master/docs)
 
-### Prerequisets
+### Prerequisites
 
 1. A supported toolchain (others might work but this is what is tested and validated)
    * Windows 10:
@@ -95,7 +95,7 @@ that a few steps should be followed.  Details of EDKII Tools can be found in the
 2. Python 3.7.x or newer on path
 3. git on path
 4. Recommended to setup and activate a python virtual environment
-5. Install the requirements `pip install --upgrade pip-requirements.txt`
+5. Install the requirements `pip install --upgrade -r pip-requirements.txt`
 
 ### Running CI