Use sh as opposed to bash in README provided shell snippets

This change updates the README to use sh instead of more specific bash
for the few code snippets we provide in there. None of the examples
require any advanced features specific to bash.

README.md

@@ -50,7 +50,7 @@ Usage
 Usage is as simple as providing the name of the respective command as a
 parameter (note that some commands are organized through subcommands,
 which are required as well), e.g.:
-```bash
+```sh
 # Open the nitrokey's encrypted volume.
 $ nitrocli storage open
 
@@ -99,13 +99,13 @@ Packages are available for:
 #### From Crates.io
 **nitrocli** is [published][nitrocli-cratesio] on crates.io and can
 directly be installed from there:
-```bash
+```sh
 $ cargo install nitrocli --root=$PWD/nitrocli
 ```
 
 #### From Source
 After cloning the repository the build is as simple as running:
-```bash
+```sh
 $ cargo build --release
 ```
 
@@ -147,7 +147,7 @@ $ nix run d-e-s-o/nitrocli
 them (for various shells). A completion script can be generated via the
 `shell-complete` utility program and then only needs to be sourced to
 make the current shell provide context-sensitive tab completion support.
-```bash
+```sh
 $ cargo run --bin=shell-complete bash > nitrocli.bash
 $ source nitrocli.bash
 ```