README: Minor update

Download TBB release using `$TBB_VERSION`.

Author: hsbadr

README.md

@@ -111,7 +111,7 @@ directory `/path/to/math/lib/tbb` as absolute path to the system-wide
 Intel TBB
 ---------
 
-`math` now supports the new interface of Intel TBB and allows using external library (e.g., with [`oneTBB`](https://github.com/oneapi-src/oneTBB) or the system TBB library), using `TBB_LIB` and `TBB_INC` environment variables.
+`math` supports the new interface of Intel TBB, can be configured to use an external copy of TBB (e.g., with [`oneTBB`](https://github.com/oneapi-src/oneTBB) or the system TBB library), using the `TBB_LIB` and `TBB_INC` environment variables.
 
 To build the development version of `math` with [`oneTBB`](https://github.com/oneapi-src/oneTBB):
 
@@ -121,7 +121,7 @@ For example, installing [`oneTBB`](https://github.com/oneapi-src/oneTBB) on Linu
 ```bash
 TBB_VERSION="2021.1.1"
 
-wget https://github.com/oneapi-src/oneTBB/releases/download/v2021.1.1/oneapi-tbb-$TBB_VERSION-lin.tgz
+wget https://github.com/oneapi-src/oneTBB/releases/download/v$TBB_VERSION/oneapi-tbb-$TBB_VERSION-lin.tgz
 tar zxvf oneapi-tbb-$TBB_VERSION-lin.tgz -C $HOME
 
 export TBB="$HOME/oneapi-tbb-$TBB_VERSION"