Merge bitcoindevkit/bdk#1291: doc(chain,esplora): minor documentation improvements

b6a58d4f9bab13be0f3518f556936fa77a6b7b39 doc(chain,esplora): minor documentation improvements (vmammal)

Pull request description:

  * Corrects an awkward use of the word 'an'
  * Adds missing backticks, brackets in a few places

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  danielabrozzoni:
    ACK b6a58d4f9bab13be0f3518f556936fa77a6b7b39

Tree-SHA512: 42f12f218a9849b6fc1c8da28c4d41ca86fcc9ea52311d28f67fadbec030b6cb6986774d89a30e7a2b5ee569b08c7cfa2af71bbb9b06efacfa685dbe95820f73