[doc] Fix typos for parallelism.

doc/doxygen/pages/external-libs.md

@@ -40,7 +40,7 @@ Download: https://gitlab.dune-project.org/extensions/dune-spgrid.git
 
 ## Parallel computing (shared memory)
 
-If one of the following libraries is installation, multi-threaded code is enabled in DuMux:
+If one of the following libraries is installed, multi-threaded code is enabled in DuMux:
 
 * `OpenMP`: OpenMP usually comes with the compiler
 * `OneTBB`: TBB, see https://github.com/oneapi-src/oneTBB
@@ -52,7 +52,7 @@ or by modifying it in the CMake cache (in `dumux/build-cmake` run `ccmake .`).
 When running `dunecontrol` or CMake, the output contains a line like
 
 ```
--- Dumux multithreading backed: TBB
+-- Dumux multithreading backend: TBB
 ```
 
 which in this case shows that the selected backend is TBB.

doc/doxygen/pages/parallel.md

@@ -89,11 +89,11 @@ Some parts of Dumux application can exploit parallelism with the shared memory m
 
 Multithreading is enabled if a multi-threading backend is found. Currently, we support one of `OpenMP`, `TBB`,  C++ parallel algorithms, `Kokkos`. The backend is selected by `CMake` during configure and stored in the variable `DUMUX_MULTITHREADING_BACKEND`. In the `CMake` terminal output during `dunecontrol` (see @ref installation), you may see
 
-    -- Dumux multithreading backed: TBB
+    -- Dumux multithreading backend: TBB
 
 or
 
-    -- Dumux multithreading backed: Serial
+    -- Dumux multithreading backend: Serial
 
 if no suitable backend could be found. You can switch backends after DuMu<sup>x</sup> has been configured by running in the build folder (e.g. `dumux/build-cmake`)