(grammar) Standardize useage of possesive form of "decryption materials"

"Decryption materials" refers to a set of materials, and is plural. In most cases when we refer to an element within the set, we use the phrase "the decryption material's [x]". This is inconsistent, as nowhere else is "decryption materials" singular.

There is place place where we use the phrase "the decryption meterials' [x]". This maintains the plural "materials", however resident grammar person @juneb suggests plural possessive should be avoided.

```
% grep -nr "material's" framework
framework/raw-rsa-keyring.md:158:- The output of RSA decryption is set as the decryption material's plaintext data key.
framework/structures.md:161: included in this decryption material's [algorithm suite](#algorithm-suite).
framework/structures.md:174:included in this encryption material's [algorithm suite](#algorithm-suite).
framework/structures.md:214: included in this decryption material's [algorithm suite](#algorithm-suite-1).
framework/structures.md:227:included in this decryption material's [algorithm suite](#algorithm-suite-1).
% grep -nr "materials'" framework
framework/raw-aes-keyring.md:146:The keyring MUST attempt to serialize the [encryption materials'](structures.md#encryption-materials)
framework/raw-aes-keyring.md:177:The keyring MUST attempt to serialize the [decryption materials'](structures.md#decryption-materials)
```

Start with the grep results in framework/raw-rsa-keyring.md, framework/structures.md, and framework/raw-aes-keyring.md, then review the surrounding references to encryption and decryption materials. Standardize the possessive wording consistently across these occurrences, following the issue's preference to avoid plural possessives, and verify no inconsistent forms remain with the grep searches.

Written by the indexing model from the issue text.